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The level of altruism that humans display is an anomaly in the animal world. Most species don’t interact peacefully with strangers every day or build large, stable societies that rely on cooperative behavior between unrelated individuals. Although there are animals that show altruistic behavior toward their relatives or breeding partners, we still don’t know how humans managed to develop the extreme level of cooperation between strangers needed to build and maintain our societies.

A paper in Nature hints that religion may be one of the keys to understanding this cooperation. The paper's authors suggest that, as people started to believe in gods who see everything and punish wrongdoing, they may have had more motivation to behave nicely toward strangers. They also suggest that beliefs in more powerful gods might widen the circle of cooperation: the more all-knowing your deity, the farther away people can be from you and still benefit from your cooperation.

To test this idea, the authors studied nearly 600 people with a wide range of beliefs from countries around the world. The beliefs included predominant world religions such as Christianity and Hinduism but also local traditions like ancestor worship, animism, and belief in supernatural entities like saints or ghosts. After answering detailed questions about what they believed, participants played a game to assess how they would act toward other people.

The game was essentially a test of how likely a person is to cheat others when no one is looking. Participants were given two cups and a pile of 30 coins to be allocated to the two cups. The participants were divided into two groups based on who the coins in each cup went to: one group could allocate coins to either themselves or a distant member of the same religion (co-religionist), while the other group could allocate coins to either a local or distant co-religionist.

In the game—which was played individually—the participant was instructed to pick up a coin and mentally choose a cup to put it in. They then throw a six-sided die with three sides of one color and three sides of a different color. If one color comes up, they put the coin in the cup they chose. If the other color comes up, they have to put it in the cup they didn’t choose.

Obviously, participants can cheat; that’s the point. If people played perfectly honestly, the researchers would be able to take all the results from all the people who played, combine them, and get data that looks basically like what they’d collect from just throwing the die over and over again and putting the coins where the die dictated.

If people cheat, the pattern looks different. The more likely they are to sneakily pretend to “change their mind” about which cup they initially decided on, the more the pattern will diverge from a random one. This allows an insight into how much people are prepared to cheat to benefit one of the cups.

The researchers found that people who believed in more moralistic, punishing, all-knowing gods were less likely to unfairly benefit themselves or local co-religionists over distant co-religionists. Other religious features, like being rewarded for good behavior, didn’t appear to make a difference; the outcome rested on how much their beliefs involved a god that was watching and prepared to punish them for being immoral.

There are questions that this particular study doesn’t answer. For instance, it can’t tell us about whether people would cheat a local co-religionist in favor of themselves, because it didn’t test that combination. It also can’t tell us how those who believe in more punishing gods would allocate coins to distant people who don’t believe in the same god. Testing whether people are altruistic toward distant people of the same group is a necessary first step; testing whether this extends to distant people of a different group is a big jump from that.

Of course, there are other societal practices and patterns that probably contributed to building large, cooperative societies, but this evidence suggests that religion—particularly belief in a god who can see you cheating and might punish you for it—likely played an important role in developing the social structures that we see today.